Led emits heat, just not going towards the tank. It emits slightly less heat, most at base of led, hence the heatsinks and/or fans found in most led fixtures.
T5 has good spectrum, if you can filter off the infrared, t5 is better than led. If you get good brands t5 vho and you managed to filter off the infrared, you get power efficiency that is similar to led and contributes as little heat as led to the tank.
The reason why leds appear brighter than t5 is simply because it is a point source of light and is wavelength specific, try looking at violet led and it is dim. Our eyes are sensitive to wavelengths non uniformly.
If you are comparing intensity per area of bulb, led will win hands does. If you are comparing intensity per wattage, t5 is comparable and it also emits a wider spectrum, including yellow, red and green (for tri-phos tubes)